AP-1 as a Therapeutic Focus in CLL

A recent study published in HemaSphere provides new insight into how chronic lymphocytic leukemia (CLL) cells exploit the tumor microenvironment (TME) to drive therapy resistance and disease progression.¹ Using a fully human 3D cell culture model that closely mimics in vivo bone marrow niches, the researchers demonstrated that CLL B cells, in cooperation with autologous T cells, reprogram bone marrow–derived stromal cells (BMSCs) and activate the AP-1 transcription factor complex to gain survival advantages.

“Chronic lymphocytic leukemia (CLL) cells actively reprogram their tumor microenvironment (TME) to promote drug resistance and tumor progression. Tumor cell survival critically depends on heterotypic communication with benign cells in the microenvironment, particularly bone marrow-derived stromal cells (BMSCs),” the authors explain. Researchers found that compared to the 2D structure,² in the “core regions of the 3D structure, CLL B cells show an even greater survival advantage, with reduced cell death compared to those in the periphery.”

The new research points to the potential of AP-1 and stromal interactions as promising targets to overcome resistance and improve outcomes in CLL. | Image credit: jarun011 - stock.adobe.com

Analysis of cellular populations within the 3D scaffold showed that B cells represented over 40% of cells in the peripheral regions and more than 32% in the core, while T cells made up about 11% in both compartments.¹ B cells in the core region exhibited a significantly greater survival than those in the periphery (P < .01), evidenced by lower rates of apoptosis and enhanced resistance to multiple therapeutic agents. 

T-cell phenotyping revealed that core regions harbored a more activated yet exhausted population. CD137 expression was significantly higher in both CD4⁺ and CD8⁺ T cells in the core, alongside increased levels of CD40L, CD25, and Bcl6 in CD4⁺ cells. Exhaustion markers PD-1, TIM-3, and LAG-3 trended upward, while cytotoxicity markers Granzyme B and Perforin were elevated in core CD8⁺ cells. Regulatory T cells and follicular helper T cells were also more abundant.

When isolating CLL B cells from core versus peripheral zones, bulk RNA sequencing identified 715 genes upregulated in the core-localized CLL cells versus only 74 genes in the periphery. The core zone showed marked increases in genes associated with survival, immune regulation, and cell adhesion, as well as prominent upregulation of the AP-1 transcription factor family, which is strongly implicated in cell proliferation and therapy resistance.

Further single-cell RNA sequencing of BMSCs exposed to CLL cells uncovered a unique, contact-induced subpopulation, termed contact-induced BMSCs (ciBMSCs). These ciBMSCs displayed distinctive gene expression patterns, with high levels of immunosuppressive and inflammation-related genes, as well as markers such as BTK, PD-L1, WLS, and POU3F1. Investigators further explain that high levels of BTK in ciBMSCs could promote cancer cell growth and progression, while upregulation of PD-L1 and WLS suggested mechanisms for local immune suppression. Some changes persisted even after CLL removal, indicating both reversible and stable reprogramming.

The study further showed that AP-1 inhibition disrupts these protective niches.

Treatment with SR11302 and T5224, specific AP-1 inhibitors, significantly increased apoptosis in core CLL cells, although treatment with the combination of AP-1 inhibitors and enzataurin had the most substantial effect. Importantly, inhibition reduces PD-L1 and WLS expression in BMSCs, directly affecting the ciBMSC subset.

“The crucial role of the AP‐1 complex in CLL cell survival and the regulation of BMSC gene expression underscores its potential as a therapeutic target,” the investigators suggest. Adding that “Our findings provide important insights into the cellular organization and molecular mechanisms underlying drug resistance in CLL.”

This research emphasizes the potential for therapies that target not only malignant B cells but also the reprogrammed stromal compartment, pointing to the potential of AP-1 and stromal interactions as promising targets to overcome resistance and improve patient outcomes.

References

1. Lindacher J, Hartebrodt A, Dingfelder J, et al. 3D Tumor microenvironment interaction reveals AP-1 complex regulation and contact-mediated reprogramming of bone marrow stromal cells in chronic lymphocytic leukemia. Hemasphere. 2025;9(8):e70199. doi:10.1002/hem3.70199

2. Jensen C, Teng Y. Is it time to start transitioning from 2D to 3D cell culture? Front Mol Biosci. 2020;7:33. doi:10.3389/fmolb.2020.00033